1. Field of the Invention
This invention relates to a grease composition, which is adapted for use in bearings of spindle motors ordinarily employed in peripheral information devices such as HDD (hard disk drive) and FDD (floppy disk drive) memories, CDD (compact disk drive), MOD (magneto-optical disk drive) and the like of computer systems, and VTR (video tape recorder).
2. Description of the Prior Art
In general, performances required for a bearing grease composition, which is employed in peripheral information devices such as HDD (hard disk drive) and FDD (floppy disk drive) memories, CDD (compact disk drive), MOD (magneto-optical disk drive) and the like of computer systems, and VTR (video tape recorder), include a reduced degree of grease dusting or scattering, a small torque, an excellent acoustic property, a long life, and the like.
Especially, with these information devices usually employed in a clean environment, it has been often experienced that a gaseous oil or fine particles of a grease scattered from the inside of a bearing at the time of rotations often cause the surfaces of a disk or the like to be contaminated, resulting in malfunction of the device. Thus, it has been accepted as the most important how to suppress the amount of the scattered oil or grease. Extensive studies have been made on this area.
In recent years, attention has been paid, as a very important problem to solve, to a phenomenon called fretting. More particularly, when ball bearings particularly used in information devices undergo vibrations at a low frequency of about 5 to 10 Hz caused during the course of carrying out and in of information devices or at the time of carrying such devices, race faces in contact with balls inside a bearing suffer damages and are degraded.
When fretting takes place, the acoustic characteristics of the ball bearing not only are worsened, but also adversely influence the performance of the information device.
It is an object of the invention to provide a grease composition for bearings of information devices, which ensures not only a high performance and a long life while suppressing an amount of an oil or grease scattered from the bearing, but also such an effect of not causing the fretting phenomenon.
It is another object of the invention to provide a grease composition for bearings of rolling of information devices which can satisfy properties required for the grease composition used in peripheral information devices such as HDD (hard disk drive) and FDD (floppy disk drive) memories, CDD (compact disk drive), MOD (magneto-optical disk drive) and the like of computer systems, and VTR (video tape recorder), i.e. a reduced degree of dusting or scattering, a small torque, an excellent acoustic property and a long life during the course of high revolutions, and also can suppress the fretting phenomenon from occurring.
It will be noted that the term xe2x80x9cbearing of an information devicexe2x80x9d is intended to mean a bearing of a spindle motor, which is used in the peripheral information devices such as HDD (hard disk drive) and FDD (floppy disk drive) memories, CDD (compact disk drive), MOD (magneto-optical disk drive) and the like of computer systems, and VTR (video tape recorder).
The above objects can be achieved, according to the invention, by a grease composition, which comprises:
a carbonate compound of the following general formula (i) serving as a base oil 
xe2x80x83wherein R and Rxe2x80x2 may be the same or different and independently represent a branched alkyl group having from 13 to 15 carbon atoms;
a lithium soap serving as a thickener; and
at least one organomolybdenum compound selected from the group consisting of a molybdenum dithiophosphate of the general formula (ii) 
xe2x80x83wherein R1, R2, R3 and R4 independently represent an alkyl group having from 1 to 24 carbon atoms or an aryl group having from 6 to 30 carbon atoms, and a molybdenum dithiocarbamate of the general formula (iii) 
xe2x80x83wherein R5, R6, R7 and R8 independently represent an alkyl group having from 1 to 24 carbon atoms, and X represents O or S. This grease composition is excellent in characteristics for use in a bearing of information devices and does not cause any fretting phenomenon to occur.
When a carbonate compound of the following general formula (i) is used as a base oil, 
wherein R and Rxe2x80x2 may be the same or different and independently represent a branched alkyl group having from 13 to 15 carbon atoms, the resultant grease composition exhibits excellent characteristics such that while characteristic properties required for information devices, e.g. a reduced degree of dusting (scattering) of the grease, a small torque, an excellent acoustic characteristic, a long life, and the like, can be imparted to the device, fretting is not caused to occur.
The carbonate used in the present invention is represented by the formula (I) in which R and Rxe2x80x2 may independently be a branched alkyl group having from 13 to 15 carbon atoms and may be the same or different. Specific examples include those carbonates of the following general formulas (v), (vi) and (vii) 
In the practice of the invention, an alkyl diphenyl ether or a polyol ester may be used in combination as base oil component.
The grease composition of the invention should preferably comprise 70 to 95 parts by weight of the carbonate and 5 to 30 parts by weight of a lithium metal salt.
If the lithium metal salt is less than 5 parts by weight, a mixing consistency becomes so low that the resultant mixture is unfavorably apt to escape or scatter when a bearing filled therewith is rotated, with the possibility that HDD, FDD and the like are contaminated therewith.
On the other hand, when the content exceeds 30 parts by weight, the resultant mixture becomes so hard that the fluidity of the resulting grease composition inside a bearing becomes poor. This may unfavorably lead to a failure in lubrication.
Further, the grease composition for a bearing of information devices according to the invention may further comprise, as a fourth component, additives ordinarily employed in the grease composition, such as antioxidants, rust inhibitors and the like.
The at least one organomolybdenum compound selected from the molybdenum dithiophosphate and the molybdenum dithiocarbamate should preferably be present in an amount of 0.5 to 5 wt %, more preferably from 1 to 3 wt %, based on 100 parts by weight of the total of the base oil and the thickening agent.
Preferred embodiments of the invention are summarized below.
(1) A grease composition for a bearing of information devices comprising:
a carbonate compound of the following general formula (i) serving as a base oil 
xe2x80x83wherein R and Rxe2x80x2 may be the same or different and independently represent a branched alkyl group having from 13 to 15 carbon atoms;
a lithium soap serving as a thickener; and
at least one organomolybdenum compound selected from the group consisting of a molybdenum dithiophosphate of the general formula (ii) 
xe2x80x83wherein R1, R2, R3 and R4 independently represent an alkyl group having from 1 to 24 carbon atoms or an aryl group having from 6 to 30 carbon atoms, and a molybdenum dithiocarbamate of the general formula (iii) 
xe2x80x83wherein R5, R6, R7 and R8 independently represent an alkyl group having from 1 to 24 carbon atoms, and X represents O or S.
(2) The grease composition as recited in (1) above, wherein the base oil is made of a carbonate compound of the following general formula (i) 
wherein R and Rxe2x80x2 may be the same or different and independently represent a branched alkyl group of the following general formula (iv) 
wherein n=13 to 15 and m=0 to 6.
(3) A grease composition for a bearing of information devices which comprises 70 to 95 parts by weight of a carbonate compound of the general formula (i) 
wherein R and Rxe2x80x2 may be the same or different and independently represent a branched alkyl group of the following general formula (iv) 
wherein n=13 to 15 and m=0 to 6, and 5 to 30 parts by weight of a lithium soap.
(4) The grease composition as recited in any one of (1) to (3) above, wherein the lithium soap is made of a lithium metal salt prepared from lithium hydroxide and a higher fatty acid having 10 or more carbon atoms or a higher hydroxy fatty acid having 10 or more carbon atoms.
(5) The grease composition as recited in any one of (1) to (4) above, wherein the thickener consists of a plurality of thickeners.
(6) The grease composition as recited in any one of (1) to (5) above, wherein at least one organomolybdenum compound selected from the molybdenum dithiophosphate and molybdenum dithiocarbamate is present in an amount of 0.5 wt % to 5 wt %, preferably from 1 wt % to 3 wt %, based on 100 parts by weight of the total of the base oil and the thickener.
(7) The grease composition as recited in any one of (1) to (6) above, further comprising a fourth component.
The invention is described in more detail by way of examples.